The present invention relates to a monitoring device for portable breathing apparatuses. Portable breathing apparatuses of this kind are used for example by divers, by fire fighters when fighting fires or generally whenever air is charged with noxious substances which make unaided breathing impossible. Portable breathing apparatuses usually consist of one or two metal bottles which are carried for example on the back of the user and in which a highly compressed oxygen gas mixture at a pressure of for example 350 bar is contained. This oxygen gas mixture is designated below, for the sake of simplification, as breathing air or simply as air. The breathing air is removed from the bottles via a shut-off valve and breathed in by the user by means of a so-called demand valve.
The problem in using breathing apparatuses of this kind is initially described by reference to the example of scuba diving:
In professional scuba diving today depths of over one hundred meters are reached and, even when diving as a hobby, experienced divers go down to considerable depths.
As the depth of water increases, the hydrostatic pressure acting on the diver becomes greater, which leads to the body tissues absorbing a relatively high amount of inert gases, that is to say in particular nitrogen. During resurfacing and the associated pressure reduction this process is reversed. If the pressure reduction occurs more quickly than the gas, which is being released, can be carried off and breathed out, decompression sickness occurs which in less severe cases leads to temporarily-health but in more severe cases can lead to permanent damage to health and even to death. In order to prevent a rapid release of the inert gases, when returning to the surface after a relatively long time spent at a relatively great depth divers must therefore remain at specific depths for relatively long resurfacing interludes which are referred to as so-called decompression stops. The duration of the necessary decompression stops is difficult to calculate since the human body has a multiplicity of different types of tissue which differ both with respect to the saturation and desaturation behavior as a function of the diving depth and duration of diving and also with respect to the medical hazard. Therefore, divers usually use diving tables in which the decompression times are given as a function of the diving depth reached and the duration of diving or they use diving computers in which the saturation and desaturation behavior of a selected number of types of tissue are mathematically simulated and the decompression times thus calculated are displayed to the driver via corresponding display devices.
A summary of the problems of decompression is given, for example, by the publication by A.A. Buhlmann: Decompression--Decompression Sickness, Berlin, Heidelberg, New York, Tokyo 1984, ISBN 3-540-13308-9, specifically in particular pages 1-62 for the medical aspect and pages 63-67 for the decompression calculation. Pages 68-82 contain decompression tables for divers.
Therefore, before the diver undertakes such a dive he must ensure that the air supply he carries is adequate for the planned bottom time and for the ascent time.
However, determining the required air supply is faced with considerable difficulties: the amount of air taken in by the diver per minute is not constant but changes, for example with the physical stress. In states of fear and panic, the air consumption can increase suddenly as a result of so-called hyperventilation. Furthermore, the amount of air removed is, of course, dependent on the respective ambient pressure and thus depends on how deep the diver is diving.
Therefore, the diver requires a monitoring device to be able to estimate the actual air consumption and the remaining possible bottom time under water.
Currently, in order to monitor the air supply divers use manometers which are connected to the breathing apparatus via a hose and indicate the current pressure of the air supply in the container. Since the pressure drops as air is continuously removed from the bottle, the appropriately experienced diver can estimate to a certain degree how much breathing time remains.
It has also already been proposed, see for example U.S. Pat. Nos. 4,794,803 or 4,586,136, to design a monitoring device which enables the remaining time available to the diver to be determined and indicated directly from the measured bottle pressure. However, these devices have the disadvantage that they are connected to the breathing apparatus via a hose and are thus cumbersome to operate and in addition can adversely affect the freedom of maneuver of the diver.
In order to overcome this problem, it has been proposed in the Australian Patent Document AU-B-78218/87 to provide, instead of the hose, an ultrasonic transmission between the pressure sensor on the bottle and a display device. In this case, the receiving and display device is arranged on the diver's mask.
The use of such monitoring devices, in particular when they operate with a wireless signal transmission, is however only acceptable if certain safety requirements are fulfilled.
Thus, it must be ensured that the signal transmission from the transmitter to the receiver takes place correctly under all circumstances, that is to say that movements of the diver and the water, that is to say external interference etc., do not have any influence on the transmission of the measurement signal.
At the same time, it is to be borne in mind that intellectual capacities are impaired from a depth of about 30 meters by the high N2 partial pressure which has a kind of narcotic effect (nitrogen narcosis). If the monitoring device, for example, falsely indicates an excessively low air supply, this can lead to an irrational panic-like reaction even among experienced divers. Therefore, it should be ensured as far as possible that the monitoring device does not display a false signal, even for only a brief period of time.
The problems described above relating to scuba diving also apply, in a correspondingly modified manner, to the use of breathing apparatuses for fire-fighting and rescue operations and for other applications. Here too, the user requires the remaining breathing time to be specified exactly in order, for example, to be able to begin his return to safety at the correct time. Furthermore, the user here is also usually in a particularly stressed state and it must therefore be ensured that incorrect measurements and incorrect information are avoided as far as possible.